1. Field of the Invention
The present invention generally relates to instruments such as scanning electron microscopes having scanning electron beams and, more particularly, to systems that improve the resolution provided by such instruments.
2. State of the Art
Instruments such as scanning electron microscopes are increasingly being used to support manufacturing operations. For example, in semi-conductor wafer processing facilities, scanning electron microscopes are employed for on-line inspections and measurements. Because of the short wavelengths of their source electrons, scanning electron microscopes have several advantages as compared to optical microscopes. For example, scanning electron microscopes provide resolutions from about one hundred to about two hundred angstroms in semi-conductor production applications, while the limiting resolution with optical microscopes is about 2,500 angstroms (i.e., 0.25 microns). Stated somewhat differently, the practical limit to magnification with optical microscopes is about 2,000X, while magnification for scanning electron microscopes broadly ranges from about 25X up to about 300,000X. Moreover, scanning electron microscopes provide depths of field which are several orders of magnitude greater than optical microscopes.
At the high magnifications provided by scanning electron microscopes, even slight vibrations can seriously impair image quality. To alleviate the effects of structural vibrations on image quality, scanning electron microscopes are usually mounted on elastic vibration-isolating structures. Such isolating structures are relatively effective in at least partially attenuating many, but not all, structural vibrations above a few cycles per second. To further alleviate the effects of structural vibrations, it is desirable to rigidly connect the specimen stage in a scanning electron microscopes to the scanning electron microscope's chamber. In the ideal case, the stage-to-chamber connecting structure is sufficiently stiff that vibrations affect both the stage and chamber equally and, therefore, do not affect the electron beam scanning pattern. Such rigidity cannot be practically achieved, however, in scanning electron microscopes that have moveable or adjustable specimen stages.
In scanning electron microscopes with movable specimen stages, vibrations continue to be a problem in terms of loss of image resolution at high magnifications. Image degradation is especially acute when such scanning electron microscopes are used in "clean rooms" in semiconductor manufacturing facilities. The environment in clean rooms normally includes acoustic vibrations as well as structural vibrations caused by machinery and the turbulence of high flow rates of ambient air. Thus, scanning electron microscopes in such environments are exposed to high-frequency vibrations as well as low-frequency vibrations. Moreover, it has been found that acoustic vibrations are often not effectively attenuated by the elastic vibration-isolating structures which are conventionally provided to alleviate the effects of structural vibrations.
In view of the preceding discussion, it can be appreciated that a need exists for ways and means to allow scanning electron microscopes to be used at high resolutions in environments that expose the scanning electron microscopes to vibrations that are not completely attenuated by elastic supporting structures.